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Why does a rebuild stop if it encounters a URE? Couldn't it just skip that bit/sector and continue rebuilding the rest of the data? The data is still there, and a tape backup or any other backup will also have missing data at the point of that URE, yet it's not fatal.

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Why does a rebuild stop if it encounters a URE? Couldn't it just skip that bit/sector and continue rebuilding the rest of the data? The data is still there, and a tape backup or any other backup will also have missing data at the point of that URE, yet it's not fatal.

Depends what was hit.

RAID uses a much larger... lets call it 'window' of data to determine how to rebuild missing files.
Unlike say, RaidZ (ZFS) which checksums each file\block individually.

For this reason, ZFS can skip a file, and restore the rest of them.
However, while there may be a good chance nothing is lost, on RAID5, if that "bad bit of data" causes a decent size calculation error, you can't be sure what you've lost.

Honestly, there's no reason not to use ZFS these days if it's a dedicated storage machine.